The invention relates to a ribbon feed mechanism with an electric drive motor reversible for reversing the ribbon feed direction.
It is known in typewriters to transport the inked ribbon, after each depression of a, or the, type key, by one step by taking off the ribbon stepwise from a ribbon spool and winding it on the other spool. Once the ribon has run its length, the ribbon feed direction is reversed so that the ribbon is now taken off from the spool on which it was previously wound, and is wound up on the spool from which it was previously taken off. The mechanism effecting this ribbon feed and the reversal of the feeding direction is expensive from a constructional viewpoint, is sensitive to dirt, and cannot be utilized, in particular, with matrix printers, for the movements of the pins cannot yield any power for ribbon feed (to rotate a ribbon spool). Ribbon feed can be executed more simply by an electrical drive motor, especially in electrically driven printers, e.g. matrix printers, in particular since a reversal of the drive direction can be reliably attained by simple electrical switching means which, in case of an electronically controlled printer, e.g. matrix printer, require only a minor supplementation of the entire control circuit.
In a commercial matrix printer comprising a ribbon feed mechanism driven by a reversible electric motor, the latter is reversed by an electronic control circuit. The electronic control circuit responds to the current rise occurring once the inked ribbon, the ends of which are attached to respectively one ribbon spool, can no longer be wound onto the driven ribbon spool because it has been completely pulled off the other ribbon spool so that the motor can no longer execute a revolution, i.e. it is blocked, thus absorbing a current which is stronger than the operating current. In case of bistable circuits customary for such control operations, it is uncertain, and left up to coincidence, which direction of revolution the motor has assumed at the time the matrix printer is turned off and is then turned on again. Thus, no assurance was afforded that the ribbon in each case was wound completely from one spool to the other and only thereafter was taken off and again completely wound onto the first-mentioned spool. The result was wear and tear nonuniformly distributed along the length of the ribbon and thus a shorter lifetime of the ribbon; for a partially used-up ribbon can no longer be utilized even if it still contains quite well usable areas. Therefore, one had either to tolerate the shorter useful life of the ribbon, or one had to take care that the printer remained turned on at all times, thus having to contend with consumption of power even during nonuse.
The invention is based on the object of providing a ribbon feed mechanism of the type mentioned hereinabove wherein the ribbon, after the printer equipped with the mechanism has been turned off and then on again, is always advanced reliably in the same direction in which it was last advanced before the printer was turned off, so that the ribbon feed direction in all cases is reversed only if the ribbon has been unwound down to one of its ends.